Prior art methods for control of spatial distribution of layer thickness in vacuum deposited coatings have been devised primarily with one of two goals. One of these goals is the formation of a layer or layers of uniform thickness on a substrate. The other is the forming a layer or layers the thickness of which varies in a predetermined manner over the surface of a substrate.
One of the earliest, and still-used, methods of achieving layer thickness uniformity involved rotating a flat, or slightly dome-shaped, substrate holder, to which one or more substrates could be attached, about a central axis offset from the a source of vapor for forming the layer. Using this method it is possible to achieve layer thickness over the substrate holder which is approximately uniform, the extent of the uniformity determined by primarily by the ratio of the offset of the rotation axis to the height of the substrate holder above the vapor source. The smaller the substrate holder relative to the height above the source, the greater the uniformity is achievable. This method has been embellished by additionally providing rotation of the substrate holder about an axis offset from the center of the substrate holder, such that the substrate holder is doubly rotated in a "planetary" fashion relative to the source. Any sub-area of the substrate holder receives vapor from the source at a range of incidence angles during the course of deposition. One important advantage of this "planetary" method is that non-uniformity of vapor output from the vapor source is compensated for. The planetary method, may be adapted for applying an approximately uniform thickness layer to a substrate which is flat, or to a substrate, such as a lens element or a dome, which has a finite surface curvature.
The planetary method is sometimes further embellished, either for providing more exact uniformity or for coating non-flat surfaces, by locating a leaf-type mask of a suitable profile, between the substrate holder and the vapor source. This mask is typically placed relatively close to the substrate holder, partially shielding the substrate holder from vapor from the source. The mask is arranged such that rotation of the substrate holder about its central axis rotates the substrate holder relative to the mask. This combination mask/planetary method has the disadvantage that a complex mechanism is required to achieve what is essentially a triple-rotation scheme of substrate holder and mask. Such a complex mechanism may be mechanically unreliable at the high temperatures (up to about 400.degree. C.) to which substrates are sometimes heated in deposition of certain coating materials, for example indium tin oxide (ITO). Complex rotating mechanisms are also known to agitate debris in coating apparatus including such mechanisms. This may lead to the inclusion of scattering or defect sites in a deposited layer.
There are several layer deposition applications in which it is required to provide one or more layers which has a thickness which is spatially varied in a predetermined manner. One such application is providing multilayer optical interference device which has an operating wavelength range which varies spatially over a substrate on which the filter is deposited. One example of such a device is a narrow-band filter deposited on a disc shaped substrate, with the transmission wavelength varying with angular position around the disc. This type of device is generally referred to as a circular variable filter (CVF) and may be used in conjunction with a light source, slit, and detector to form a spectrophotometer. The device would require a layer thickness ratio between thickness extremes of about 2:1 to cover even only the visible spectrum.
One method for making this and similar device types is disclosed in U.S. Pat. No. 3,442,572. The method involves depositing coating material on a disc-shaped substrate by the use of first and second masks. The first and second masks and the substrate form three elements, and at least two of the elements are rotated with respect to each other, and to the third element, to cause the material to be deposited on the substrate so that the optical thickness of coating layers varies with azimuthal angle over a segment of the substrate. While elegant in concept, this method requires two masks and the necessary relative rotation of those masks for each substrate to be coated. This makes mass production of such devices by this method a daunting prospect.
Another method of forming layers having a predetermined thickness variation is described, by J. R. Kurdock and R. R. Austin in "Physics of Thin Films Vol. 10", pp 261-308, Academic Press (1978). Here, a mask having a spatially varying transmission for coating vapor is placed between a source and a substrate. The mask is prepared by plating a half-tone pattern of metal dots on a wire mesh or wirecloth screen using a complex, computer controlled lithographic process. The mask is essentially "two-dimensional" inasmuch as it is formed from a very thin material and spatially varying layer thickness is achieved by spatially varying the size of areas of the mask through which coating vapor can pass. These areas having a width significantly greater than the thickness of the material.
This method was devised primarily for aspherizing or correcting a surface of an optical element by addition of material to the surface. The description, however, also discloses that layers of spatially variable thickness produced by the method are useful for other applications. Applications include forming a transparent conductive coating having a spatially varying thickness for uniformly heating an irregular shaped window; providing a filter or antireflection coating for a window on which useful light is incident at incidence angles which vary according to the position on the window on which light is incident; and for providing a bandpass filter with a position dependent passband wavelength.
The method has several disadvantages. One disadvantage is that, as described, the mask making process takes several hours. Another disadvantage is that the mesh of the mask is not rigid and must be stretched in a rigid frame to maintain a flatness necessary to ensure uniform spacing of a the mask from a substrate being coated. Further, a preferred wirecloth is formed from 0.0037 inch diameter wires in a 70 mesh weave, which suggests that such a mask would have a maximum vapor transmission of only between about fifty and sixty percent and may become blocked by coating material during deposition of filter having multiple thick layers.